1. Field of the Invention
The invention relates to semi-conductive compositions useful for extrusion coating wires and cables. More specifically, the semi-conductive compositions of the invention are linear low density polyethylenes containing two or more different carbon blacks which impart a superior balance of properties, including dispersibility, moisture vapor transmission, environmental stress crack resistance, thermo-oxidative stability and physical/mechanical properties.
2. Description of the Prior Art
Linear low density polyethylene (LLDPE) is widely used in the wire and cable industry as a coating in electrical and telephone applications where flexibility, strength, low brittleness temperature and high resistance to abrasion are required. LLDPE has been found to be particularly useful for overhead and underground medium and low voltage power cable constructions. Power cables are multi-layer constructions of specifically formulated compositions which impart the requisite characteristics to the final construction and LLDPE-based compositions have been widely used for the conductor shield layer, the insulation shield layer and the exterior jacketing layer for such constructions. Conductor shield and insulation shield layers are semi-conductive and typically have large amounts of conductive carbon black compounded with the LLDPE. LLDPE-based jacketing compositions are usually non-conductive and generally contain about 2.5 to 3 percent conventional carbon black to provide protection against ultraviolet radiation.
While the use of smaller particle size carbon blacks generally provides increased electrical conductivity, these small particle size, high surface area blacks are more difficult to disperse in plastic materials. This is particularly true when using LLDPEs which are widely recognized as some of the more difficult polyolefin resins to process due to their narrow molecular weight distribution. Dispersibility of small particle size conductive blacks in LLDPE is even more of a problem when incorporating the higher black loadings typically required, e.g., 10-35 weight percent, for the manufacture of conductive compounds.
It would be highly useful, therefore, to develop LLDPE semi-conductive formulations having improved processability and extrudability for wire and cable applications. It would be even more advantageous and desirable if this could be accomplished while retaining or improving conductivity and the other properties generally considered to be essential for wire and cable applications. These and other advantages are realized with the LLDPE compositions of the present invention wherein a mixture of conductive carbon blacks with specific characteristics is utilized.
U.S. Pat. No. 5,733,480 discloses polyolefin compositions containing a mixture of conductive blacks having different structures useful for the extrusion of mono- and multi-layer films and coatings. The ""480 compositions comprise conventional low density polyethylene (LDPE) resin having a density of 0.910-0.935 g/cm3 and 6 to 15 weight percent of the carbon black mixture. Conventional LDPE resins are produced in high pressure polymerization processes and are homopolymers with substantial long-chain branching. LDPEs are distinguished from LLDPEs which are comprised of linear molecules with no long-chain branching. LLDPEs are produced by low pressure copolymerization of ethylene and one or more C3-8 xcex1-olefins and contain only short-chain branches as a result of the incorporated comonomer. While the reference does disclose that LLDPE and higher density PEs can be blended with the LDPE, the amount of these additive resins is preferably kept below 15 weight percent of the total polyolefin component. There is no suggestion in the reference to the use of mixed blacks with polyolefin compositions where LLDPE is the sole or predominant resin component or that LLDPE compositions useful for wire and cable applications exhibiting a superior balance of properties can be achieved by the use of a mixture of specific conductive carbon blacks.
The present invention relates to semi-conductive extrusion compositions having complex viscosities at 210xc2x0 C. and 100 rad/sec from 6000 to 25000 poise and dispersion numbers from 50 to 700 psi comprising: (a) 75 to 95 weight percent, based on the total weight of the composition, of a base resin comprising linear low density polyethylene having a density from 0.890 to 0.925 g/cm3 and melt index from 0.3 to 15 g/10 min and (b) 5 to 25 weight percent, based on the total weight of the composition, of a carbon black mixture containing a major portion of a higher structure conductive carbon black and a minor proportion of a lower structure conductive carbon black. Preferably, the higher structure black has a BET surface area greater than 500 m2/g and dibutyl phthalate absorption number from 200 to 600 ml/g and the lower structure black has a BET surface area of 125 to 500 m2/g and dibutyl phthalate absorption number of 80 to 250 ml/g.
In a highly useful embodiment of the invention the base resin is a mixture of linear low density polyethylene with another polyolefin, preferably low density polyethylene. It is particularly advantageous when the linear low density polyethylene is a copolymer of ethylene with 2 to 25 weight percent butene-1 or hexene-1. Another highly useful embodiment utilizes from 0.1 to 2.5 weight percent of a stabilizer which is a mixture of a hindered phenol and a mercaptobenzimidazole compound. Preferably, the weight ratio of hindered phenol to mercaptobenzimidazole compound is 1:1 to 1:4. Preferred semi-conductive extrusion compositions have complex viscosities from 8000 to 15000 poise, dispersion numbers from 50 to 350 psi.
The present invention relates to semi-conductive LLDPE compositions for wire and cable applications. The LLDPE compositions of the invention contain a mixture of conductive carbon blacks which impart semi-conductivity and provide a balance of useful properties. The compositions are particularly useful as the semi-conductive layer in power cable constructions. In addition to having low resistivity, the semi-conductive formulations exhibit good processability and dispersion of the carbon black, low moisture vapor transmission, good low temperature properties and environmental stress crack resistance and good physical/ mechanical properties. Additionally, the compositions have good abrasion resistance and, when properly stabilized, good short and long-term oxidative stability.
To obtain compositions having the requisite properties, a base resin having LLDPE as the sole or predominant component is employed. LLDPE, which is typically produced by the copolymerization of ethylene with one or more C3-8 xcex1-olefins comonomers using transition metal catalysts in accordance with well-known processes, is characterized by linear molecules having no long-chain branching. Short-chain branching is instead present and is one of the primary determinants of resin density and physical properties.
LLDPE densities will range from 0.890 to 0.925 g/cm3 and, more preferably, from 0.905 to 0.922 g/cm3. Comonomers typically copolymerized with ethylene to obtain LLDPEs useful for the invention include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and mixtures thereof. By incorporating these comonomers, linear polymer molecules having short-chain branches along the polymer backbone are produced. The amount of comonomer will typically not exceed 35 weight percent and, most commonly, the comonomer is present in an amount from about 2 to 25 weight percent of the polymer composition. The specific comonomer or comonomer mixture used is primarily based on process compatibility and the desired resin specifications. LLDPE resins which are copolymers of ethylene and butene-1 and/or hexene-1 have been found to be particularly advantageous when formulating of the semi-conductive compositions of the invention. Also, for best processability and extrudability it is advantageous if the LLDPE have a molecular weight distribution (MWD) greater than 10. MWD is determined from the weight average molecular weight (Mw) and number average molecular weight (Mn) which are obtained by gel permeation chromatography. MWD=Mw/Mn. LLDPEs useful for the invention are widely available from commercial sources.
As previously indicated, an LLDPE meeting the above criteria can constitute the sole component of the base resin or it can be combined with other polyolefin resins. If the LLDPE is combined with another resin, the LLDPE will constitute the major component of the blend, i.e., be present in an amount greater than 50 weight percent, based on the total weight of the base resin. Any additional resin(s) utilized with the LLDPE to obtain the base resin will be present in minor amounts, i.e., comprise less than 50 weight percent of the base resin.
Polyolefin resins which can be utilized with the LLDPE include homopolymers of ethylene, copolymers of ethylene and xcex1-olefins and copolymers of ethylene and comonomers containing polar groups such as C1-4 alkyl esters of acrylic and methacrylic acids. Ethylene-propylene copolymers are representative of ethylene/xcex1-olefin copolymers which can be included in minor amounts. Copolymers of ethylene and polar comonomers, typically containing 1 to 35 weight percent and, more preferably, 2 to 25 weight percent comonomer, include by way of illustration ethylene/methyl acrylate and ethylene/n-butyl acrylate copolymers. In one highly useful embodiment of the invention, low density polyethylene homopolymers are employed to obtain the improved compositions of the invention. These LDPE homopolymers will generally have densities from about 0.910 to about 0.935 g/cm3 and a melt index (MI) from 1 to 25 g/10 min. In one preferred embodiment, the polyethylene resin is LDPE having a density from 0.915 to 0.930 and MI from 2 to 12.
When the base resin is a mixture of LLDPE and another polyolefin, the polyolefin preferably will not exceed 40 weight percent of the mixture. Generally, the polyolefin will constitute from 2 up to about 35 weight percent and, more preferably, 5 to 30 weight percent of the base resin. Highly useful semi-conductive wire and cable compositions are obtained using base resin mixtures comprising 70 to 95 weight percent LLDPE and 5 to 30 weight percent LDPE.
The base resin, whether comprised only of LLDPE or a mixture of LLDPE with one or more other resins, will have a density of 0.890 to 0.925 g/ cm3 and MI from 0.3 to 15 g/10 min. Melt indexes are determined in accordance with ASTM Test Method D 1238. Densitities are determined in accordance with ASTM D 1505. Base resins having a density of 0.905 to 0.922 g/ cm3 and MI of 0.5 to 7.5 g/10 min are particularly advantageous for formulation of the semi-conductive compositions.
To obtain the improved semi-conductive compositions of the invention a mixture of at least two different carbon blacks is employed. The compositions will contain 75 to 95 weight percent base resin and 5 to 25 weight percent of the carbon black mixture. More preferably, from about 80 to 90 weight percent base resin and 10 to 20 weight percent of the carbon black mixture are employed.
The carbon black mixture contains at least two conductive blacks of different structure. The term xe2x80x9cstructure,xe2x80x9d as employed herein, refers to the ability of the black particles to associate and form larger three-dimensional aggregates. Structure, i.e., aggregate size and shape, can be determined by transmission electron microscopy but is more commonly defined by determining the volume of dibutyl phthalate (DBP) in ml. absorbed by 100 grams of the black. Carbon blacks with DBP absorption numbers less than about 80 are generally considered to have insufficient structure for semi-conductive applications. DBP absorption numbers greater than about 80 and, preferably, greater than 100 are typically regarded as high structure blacks suitable for semi-conductive formulations.
The first conductive black, referred to herein as the first or higher structure carbon black, in the carbon black mixture has a BET surface area greater than 500 m2/g and DBP absorption number of 200 to 600 ml/100 g. A second conductive black with BET surface area 125 to 500 m2/g and DBP absorption number 80 to 250 ml/100 g, referred to herein as the second or lower structure carbon black, is combined with the first black to obtain the carbon black mixture necessary to achieve the improved results of the invention. Both the higher and lower structure blacks have mean particle sizes from about 10 to 50 nm and volatiles contents typically 2 percent or below. More than one higher or lower structure black may be used if desired.
The relative amount of the higher and lower structure blacks in the mixture can be varied; however, the first (higher structure) black generally comprises more than 50 percent of the black mixture. Most generally, the black mixture will contain 51 to 85 weight percent higher structure black and 15 to 49 weight percent lower structure black. In one highly useful embodiment of the invention, the higher structure black constitutes 55 to 75 weight percent of the black mixture with the lower structure black comprising 25 to 45 weight percent.
It is particularly advantageous if the first black has a BET surface area of 600 to 2000 m2/g, the second black has a BET surface area of 150 to 450 m2/g and the volatiles content of both is less than 1.5 percent. In one highly useful embodiment, the higher structure black has a BET surface area of 750 to 1300 m2/g and DBP absorption number of 300 to 500 ml/100 g and the second lower structure black has a BET surface area of 200 to 400 m2/g and DBP absorption number of 100 to 200 ml/100 g.
Carbon blacks of the above types are known and available from commercial sources. For example, a representative high structure black which can be used for the invention is PRINTEX [trademark] XE 2 manufactured and sold by DeGussa Corporation, Pigments Group. Typical properties for this black, referred to by the manufacturer as an xe2x80x9cextraxe2x80x9d conductive black, are: volatiles 1.2 percent, particle size 35 nm, BET surface area 1000 m2/g, and DBP absorption number 400 ml/100 g. Another higher structure black which meets the requirements of the invention is available from Cabot Corporation as BLACK PEARLS [trademark] 2000. This black has a BET surface are of 1475 m2/g, DBP absorption number of 330 ml/100 g, volatiles content of 2.0 percent and particle size of 12 nm. A useful carbon black of lower structure which can be used as the second black in the mixture is PRINTEX [trademark] L 6, also manufactured by DeGussa Corporation, Pigments Group, which has a volatiles content of 1.2 percent, particle size of 18 nm, BET surface area of 265 m2/g, and DBP absorption number of 120 ml/100 g. PRINTEX [trademark] L, available from DeGussa Corporation, Pigments Group, and having a BET surface area of 150 m2/g and DBP absorption number of 114 ml/100 g, and VULCAN [trademark] XC-72, available from Cabot Corporation and having a BET surface area of 254 m2/g and DBP absorption number of 178 ml/100 g, are examples of other conductive blacks which can be utilized as the second component in the carbon black mixture.
Uniform dispersion of the carbon black mixture in the base resin is necessary to provide the continuous path of conductive particles within the polymer matrix required to achieve maximum conductivity. Uniform dispersion is also essential for acceptable extrusion. Poor dispersion of black particles can restrict flow through dies resulting in of uneven thicknesses of extrudate and/or poor surface appearance. In extreme cases, poor dispersion of the black can result in blockage of screen packs which make it necessary to shut down the extrusion line until the blockage is removed. Processing conditions must be sufficient to adequately disperse the black but they cannot be so rigorous as to break down the carbon black structure, i.e., the aggregates. Excessive processing which breaks down the carbon black aggregates and results in diminished conductivity must be avoided.
To achieve proper dispersion of the carbon black mixture and insure acceptable conductivity and extrudability for wire and cable fabrication, the semi-conductive compositions of the invention have a specified complex viscosity and dispersion number. The compositions have complex viscosities from 6000 to 25,000 poise and, more preferably, 8000 to 15000 poise. If the complex viscosity is too high, the compositions are too stiff for acceptable extrusion. On the other hand, if the complex viscosity is too low the material does not have the necessary mechanical strength and extrudate surface quality will be poor. In a particularly useful embodiment of the invention, the complex viscosity is 9000 to 12000 poise. Complex viscosity measurements are conducted in accordance with ASTM Standards D 4065 Practice for Determining and Reporting Dynamic Mechanical Properties of Plastics and D 4440 Standard Practice for Rheological Measurement of Polymer Melts Using Dynamic Mechanical Procedures. Specifically, dynamic rheological measurements are obtained using a Rheometrics [trademark] RDA II instrument equipped with parallel plates. Measurements were made at 210xc2x0 C. with a frequency sweep from 159 to 0.0398 rad/sec and strain of 5 percent. Complex viscosities reported herein are at 100 rad/sec.
The present compositions will also have dispersion numbers from about 50 to about 700 psi and, more preferably, from 50 to 500 psi. Dispersion number is an indication of the size and distribution of the carbon black agglomerates within the polymer matrix after processing. The method used to determine the dispersion number is analogous to the screen pack plugging test wherein change in head pressure in an extruder is measured as the composition is extruded. For the test a composition containing 10 weight percent carbon black is extruded using a Haake System 90 single screw extruder with a heated die (305xc2x0 C.) with a breaker plate followed by a 60-60-325-60 mesh screen pack. Zones 1-3 in the extruder are heated to 235xc2x0 C. and the extruder is operated at a screw speed of 150 rpm. The dispersion number is obtained by subtracting the pressure reading obtained at 5 minutes from the pressure reading obtained after 25 minutes. In a particularly useful embodiment of the invention the dispersion number of the composition is from 50 to 350 psi.
While it is not necessary, depending on the particular end use application, it may be advantageous to include one or more other additives in the compositions of the invention. In general, the type and amount of additive(s) will be consistent with conventional formulation practices employed with other polyolefin wire and cable compositions. Useful additives include but are not limited to antioxidants, such as hindered phenols, aromatic amines, thioethers, phosphites and phosphonites; processing aids such as fluoroelastomers; dispersing agents, such as stearic acid, waxes, calcium stearate, aluminum stearate and zinc stearate; and the like. These additives typically do not exceed about 1.5 weight percent of the total formulation and, most commonly, are employed at levels from 0.005 percent to about 1 percent. Fillers, such as calcium carbonate, talc, mica and the like may also be included in the formulations.
In one highly useful embodiment of the invention, a stabilizer combination comprised of a hindered phenol and a mercaptobenzimidazole compound is employed at levels from about 0.01 up to about 2.5 weight percent, based on the weight of the total composition, is employed. The weight ratio of the hindered phenol to the mercaptobenzimidazole compound will range from about 1:1 to about 1:4 and, more preferably, be in the range 1:2 to 1:3. It is particularly advantageous when the stabilizer combination is utilized in an amount from about 0.1 to 1.5 weight percent.
Useful mercaptobenzimidazole compounds correspond to the formula 
where A is hydrogen or zinc, R is a C1-4 alkyl group, n is 0 to 4 and x is 1 or 2. Preferably, R is methyl and n is 0 or 1. Preferred mercaptobenzimidazole compounds based on their commercial availability include 2-mercaptotolylimidazole (MTI), 2-mercaptobenzimidazole (MBI), zinc 2-mercaptobenzimidazole (ZMBI) and zinc 2-mercaptotolylimidazole (ZMTI). MTI is particularly advantageous.
Useful hindered phenols will contain one or more substituted phenyl groups of the formula 
where R is a C1-4 alkyl group and, most preferably, a tertiary butyl group. Where more than one, 3,5-dialkyl-4-hydroxyphenyl group is present, they will be joined through a linking group and the resulting compounds will correspond to the formula 
where n is an integer from 2 to 4 and L represents the linking group.
Representative linking groups can include:
xe2x80x94CH2xe2x80x94
C (CH2OCCH2CH2) 4 xe2x80x94
It is especially advantageous when the above-identified linking moieties are substituted with 3,5-di-t-butyl-4-hydroxyphenyl groups.
Representative hindered phenol compounds of the above types include:
4,4xe2x80x2-methylenebis(2,6-di-t-butylphenol);
tetrakis[methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)methane;
1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)-benzene;
1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-s-triazine 2,4,6(1H,3H,5H)trione;
N,Nxe2x80x2-bis[3 -(3, 5-di-t-butyl-4-hydroxyphenyl)propanyl]-hydrazine;
octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate.
All of the foregoing materials are commercially available. Tetrakis[methylene (3,5-di-t-butyl-4-hydroxyhydrocinnamate)]methane and octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate are particularly useful in combination with MTI.
The following examples illustrate the invention more fully. Unless otherwise indicated, all parts and percentages reported in the examples are on a weight basis. Except for varying the amount and type of base resin, carbon black mixture and additives, all of the formulations were prepared and evaluated using the same procedures.
The formulations were compounded using a Farrel OOC Banbury mixer having a capacity of 2400 cc. All of the ingredients, i.e., polyolefin(s), carbon black(s) and stabilizer(s) were combined and the preheated (95xc2x0 F.) chamber filled with the mixture. A pressure of 40 psi was then applied with mixing (125 rpm). When flux was achieved, i.e., the temperature of the mixture in the chamber reached approximately 270xc2x0 F. (usually about 40-50 seconds), the ram was raised for 15 seconds and any material which collected in the throat of the mixer was scraped into the mixing chamber. Pressure was reapplied and mixing continued for at least 3 minutes or until the temperature reached 340xc2x0 F. The melt was then pelletized at 360xc2x0 F. using a 3.25 inch single screw extruder (L/D=23; 10 rpm) connected to an underwater pelletizer.
Complex viscosity (xcex7*), which provides a measure of processability by determining the Theological properties of molten polymers over a range of temperatures by non-resonant forced vibration techniques, was determined in accordance with ASTM Test Methods D 4065 and D 4440 utilizing a Rheometrics [trademark] RDA II rheometer equipped with parallel 25 mm diameter plates and operated at 210xc2x0 C. with a frequency sweep from 159 to 0.0398 rad/sec and strain of 5 percent. Complex viscosities are reported at a frequency of 100 rad/sec.
Dispersion number (DN) was determined utilizing a screen pack plugging procedure wherein the change in head pressure in an extruder is measured over time. The test was conducted using a Haake System 90 single screw extruder with a heated (305xc2x0 C.) die with a breaker plate followed by a 60-60-325-60 mesh screen pack. Extruder zones 1-3 were heated to 235xc2x0 C. and the extruder was operated at a screw speed of 150 rpm. Pressure readings (in psi) were taken at 5 and 25 minutes and the dispersion number is the difference between the readings, i.e., DN=Pt25xe2x88x92Pt5. For uniformity of comparison, all dispersion numbers are reported for a 10 weight percent carbon black loading. Thus, if a composition was formulated at a black level greater than 10 percent, it was let down into additional amount of the polyolefin resin to adjust the carbon black loading to 10 percent before conducting the pressure rise test and determining the dispersion number.
Conductivity was determined in accordance with ASTM Test Method D991 which measures volume resistivity of a sample. Since resistivity is the reciprocal of conductivity, lower volume resistivity values reflect improved conductivity. In some instances resistance was also measured using a Fluke digital ohm meter Model 87III.
Measurements we made at room temperature and at 90xc2x0 C. on 40 mil plaques with the electrodes 1 cm apart. Plaques used for the 90xc2x0 C. test were allowed to equilibrate at that temperature for 4 hours before testing. Flexural Modulus (1% Secant) was determined following ASTM D790. Elongation at break was determined on extruded film in accordance with ASTM D 638. Environmental stress crack resistance (ESCR) a was determined in accordance with ASTM D1693. Water Vapor Transmission rates (WVTR) were determined using ASTM F372. Thermomechanical stability of the compositions was determined by monitoring variation in torque as a heated sample is mixed under high shear. For this test, a 40 gram sample of the composition is placed in the mixing chamber of a Haake Rheomix heated to 220xc2x0 C. and operating at 60 rpm for a period of 50 minutes. Initially, the torque drops as the mixture melts but then the torque gradually increases due to chain extension reactions. The value reported is the torque at 40 minutes normalized with respect to the initial torque. The lower the torque value, the more thermally stable the composition.